Thank you all for the feedback. The first-hand experience from those with both cameras is quite valuable.

I must say that I'm giving the 6D some more thought due to the feedback from this thread. The price is certainly more attractive. But, my primary subject matter is figure skating and middle-school level sports with the 70-200 f2.8L II. Most of the time, I can shoot 1/500 or faster at ISO 1600-3200. But, there are times when I need 6400. I'd like any FF body that I get be able to handle the action and the 6D concerns me on this front.

My OOF rate with my 60D is much higher than with my 7D. I attribute this to the 7D's focusing system and it's superior ability to track subjects. The 7D seems much better at predictive focusing with AI Servo, especially when skaters don't always move in a straight line. My fear is that that the 6D will have a similarly high OOF rate as my 60D has.

Another concern is burst mode. I don't rely on it as a crutch for timing the shot, but I do use it and the buffer on the 60D will fill up on me. The 7D has no problem keeping up. Based on Canon's published max burst rates, the 6D looks very similar to the 60D. (By the way, I think their published rates are higher than true rates when using AI Servo.)

I don't expect the 6D to track moving objects as well as the 7D, but can it do so better than the 60D? Does its tracking ability fall between the 60D and 7D, if so where in between?

I did the exact same move (60D to 5D3) about a month ago. My objectives are the same (candids in low light) and i have to say that I'll never regret my cash. (Note: never tried the 6D)

I just came back from CNY celebrations and I would probably have a way lower keeper rate with my 60d...actually in many situations, the camera could have just stayed in the bag if it wasnt for the 5D. Night time fireworks with people running all around? Only 5D3 could have pulled that out...

You seem not to metion it a lot but the AF system is godlike. It helps improving both your OOF rate and your composition if you have quick fingers on the joystick and a little foretelling

I got an assignment 2 month ago,an actor from Hong Kong came to Malaysia for one of the club opening ceremony.I was asked to take picture of him naturally and has to avoid using flash.I manage to take a picture of him while he sing along the music while holding a champagne glass.

It can be done but maybe not at ISO 8000, as you said. If you use a lens with an aperture of f/1.4 , 1/60s and set the ISO to 3200 the exposure would be about the same, though the noise would be higher. With the crop factor the dof will be close to that of the original image too. I often shoot with the 7D at ISO 3200, with some clean up in post the files look quite useable. Depends on the lighting though. Good lighting helps a lot.

A 5d3 with the 24-105 is on its way. For my use I think this lens will be more versatile than the 24-70 (I or II) (and cheaper). I've got the 35 f2, 50 f1.8, and 40 f2.8 for the more light challenging events. I need to see what I can do with these lenses before considering faster zooms or primes. (But, an 85 f1.8 or a 100 f2 could be tempting down the road.)

A 5d3 with the 24-105 is on its way. For my use I think this lens will be more versatile than the 24-70 (I or II) (and cheaper). I've got the 35 f2, 50 f1.8, and 40 f2.8 for the more light challenging events. I need to see what I can do with these lenses before considering faster zooms or primes. (But, an 85 f1.8 or a 100 f2 could be tempting down the road.)

wait till you see what you can do with a 5Dmk3 iso 16,000 and a 1.4 prime it will blow your mind

When you only own crop, you really don't want believe that FF is so much better due the added expense. But, it is.

Just shot a grade school volleyball event with the 5D3 and the 70-200 f2.8 II. I shoot these events for my kids' school yearbook and upload a bunch that don't make the yearbook to share with parents. For this stuff, I prefer to save time and space by shooting JPEGs. With the 7D, I often got the shot, but I always had to post process the shoot to cleanup noise, and sometimes add some presence to the color. But, I can see now that the 5D3 will save lots of time. My shots from tonight need no post work, save for cropping on a handful of images. Lack of noise was amazing.

With volleyball, you need to aim and shoot quick. There's really no tracking of a subject. Just pick the player who looks ready to hit the ball, aim, shoot. I've been impressed with the 7D's ability to lock on quick, so far the 5D3 is just as quick, maybe a tad quicker because it offers a larger expanded point focussing option. I had a shorter lens on the 7D and used it for a few shots. All of a sudden, the 7D felt old. A pity, really.

All the discussion in this forum about the 5D3 in low light has me intrigued. I'm particularly interested in a 5D3 w/24-105 f4L IS as an upgrade path for a 7D w/17-55 f2.8 IS. (For low light candids, I'm often shooting at 1/30 second and find IS to be a must.)

I know that the 5D3 offers greater color depth and that full frame is typically sharper than crop. I also understand that the 5D3 offers about a 1.3 stop advantage in noise over the 7D. But, I'm comparing a crop body with a 2.8 lens to the full frame with a 4.0 lens and this noise advantage drops to about 1/3 of a stop.

Now the question. A 2.8 lens lets in more light than a 4.0, which is more light for the AF system to lock in. So which system can lock in on focus at lower light -- the 5D3 w/24-105 f4L IS or the 7D w/17-55 f2.8L IS?

You might also consider a 6D, with a 24-70 f/2.8 lens. Your cost would be similar, if not less, than a 5D3/24-105 kit (depending on which 24-70 you choose).

I feel the 6D is superior to the 5D3 for low light. I'm not alone. The 5D3 is best for slightly more than low light, and with very fast, erratic subjects. You won't be achieving your goal with the 24-105 lens. It's a great lens, but not in low light. It also doesn't AF quickly even at noon on a sunny day, no matter what body it's on. I personally would buy a 6D and the Tamron 24-70.

As for the crop factor...I also upgraded from a crop camera (50D). I've only had my 6D a week, but have shot over 1000 pictures, still haven't tried all of my lenses. All I can say is, what the full frame fanboys have been saying is true: A cropped image done with either the 6D or 5D3, will be more detailed than you think it will. It will not be as detailed as a crop body in good light, but it won't be 1.6x behind. It will be about 1.25x to 1.35x behind in good light, and about 1.1x behind in light requiring up to ISO 2000 or so. Above that, it will be ahead. These differences have become negligible from a practical standpoint, in my opinion. I no longer see a need for a crop camera (and I never thought I would not. I will be very sad to see mine go).

This evening I shot an image at ISO 8000 with my Voigtlander 58mm f/1.4, in dim outdoor light, with all in-camera NR turned off. I had it closed to about f/10. The detail is beyond anything I have ever seen short of a D800 paired with whatever their choice of sharpest lens could be, but at ISO's below 1600 for the D800. The luminance noise in the shot I'm describing, is as low as my 50D at perhaps ISO 800 or 640. The chrominance noise is similar. It's there, but with very slight NR in post, it's gone, and all the detail remains. The color depth is surprisingly nice, though the lens makes a big difference. You need a lens that is fantastic at color rendition, to make full use of the 6D.

I'm blown away by this. Yes, the physical size of the 6D, makes it seem like a toy worthy of derision by the 5D3 or 1DX crowd. It's not a toy. What it is, is what 5D2 buyers five years ago, wish they could have bought instead...and for 40% less cost to boot!

All I can say is, what the full frame fanboys have been saying is true: A cropped image done with either the 6D or 5D3, will be more detailed than you think it will. ... I no longer see a need for a crop camera (and I never thought I would not...

I find it interesting that in many cases, those arguing for the 'reach advantage' of APS-C have used only APS-C. You're far from the first to come to the conclusion you have, after adding a FF camera to your kit.